Electron-pair acceptor/donor concept

A vast generalization beyond the Bronsted—Lowry acids and bases concepts is the concept of a Lewis29 base (an electron pair donor) and a Lewis acid (an electron pair acceptor). This concept has been used extensively in all branches of chemistry. In physical organic chemistry, quantities of the type pA = —logio[A] have used extensively to study reactivities—for example, in the Hammett equation. 

It was G. N. Lewis who extended the definitions of acids and bases still further, the underlying concept being derived from the electronic theory of valence. It provided a much broader definition of acids and bases than that provided by the Lowry-Bronsted concept, as it furnished explanations not in terms of ionic reactions but in terms of bond formation. According to this theory, an acid is any species that is capable of accepting a pair of electrons to establish a coordinate bond, whilst a base is any species capable of donating a pair of electrons to form such a coordinate bond. A Lewis acid is an electron pair acceptor, while a Lewis base is an electron pair donor. These definitions of acids and bases fit the Lowry-Bronsted and Arrhenius theories, and cover many other substances which could not be classified as acids or bases in terms of proton transfer. 

In 1923 the American chemist G.N. Lewis provided a broad definition of acids and bases, which covered acid-base reactions not involving the traditional proton transfer an acid is an electron-pair acceptor (Lewis acid) and a base is an electron-pair donor (Lewis base). The concept was extended to metal-ligand interactions with the ligand acting as donor, or Lewis base, and the metal ion as acceptor, or Lewis acid. 

Lewis acids are electron pair acceptors and Lewis bases are electron pair donors. However, according to the Hard and Soft Acids and Bases (HSAB) concept [17], Lewis acids are classified into hard and soft acids, while Lewis bases are classified into hard and soft bases. Hard acids interact strongly with hard bases, soft acids with soft bases. 

The Lewis acid and Lewis base concept explains the majority of reaction chemistry that we are familiar with. Lewis acid/base reaction chemistry concerns electron pair donors, electron pair acceptor, anions, cations, lone-pairs etc.) [6,7]. 

General definitions of acids and bases were given by G. N. Lewis as an extension of the concept of the electron-pair covalent bond an acid is an electron-pair acceptor, a base an electron-pair donor. All Lewis bases are Bronsted bases—an unshared electron pair is required to accept a proton. H+ is an acid in the Lewis sense since it can accept an electron pair from a base such as NH3, but LLO and NH4... 

In Chapter 5, Lewis bases (electron pair donors) were classified as nucleophiles and electron pair acceptors were designated as electrophiles or Lewis acids. These concepts will now be used to describe coordination complexes of metals. 

According to the Lewis definition, a base is a substance that can furnish an electron pair to form a covalent bond, and an acid is a substance that can take up an electron pair to form a covalent bond. Thus an acid is an electron-pair acceptor and a base is an electron-pair donor. This is the most fundamental of the acid-base concepts, and the most general it includes all the other concepts. 

SECTION 16.11 The Lewis concept of acids and bases emphasizes the shared electron pair rather than the proton. A Lewis acid is an electron-pair acceptor, and a Lewis base is an electron-pair donor. The Lewis concept is more general than the Bronsted Lowry concept because it can apply to cases in which the add is some substance other than H. ... 

Nucleophilicity and electrophilicity are closely related to Lewis basicity and acidity, respectively. Nucleophiles are Lewis bases (electron-pair donors) and electrophiles are Lewis acids (electron-pair acceptors). Now, as discussed previously, nucleophilicity is measured in terms of the rate of a nucleophilic attack, so it s a kinetic concept. Basicity, on the other hand, is measured in terms of the equilibrium constant for protonation (or for association with some Lewis acid), so it is a thermodynamic concept. Another difference is that. 

It is clear that atoms other than hydrogen can be electron deficient and function as electron pair acceptors. Can a carbon atom function as a Lewis acid The answer is yes, if the definition is modified somewhat. Various reactions generate carbocation intermediates (see 55 and 58) and a Lewis base can certainly donate electrons to that positive carbon. A species that donates electrons to carbon is called a nucleophile (see Section 6.7), so an electron donor that reacts with 55 or with 58 is a nucleophile. In addition to carbocations, which are charged species, the carbon atom in a polarized bond is electron deficient, and a nucleophile could donate electrons to the 6+ carbon. This is the basis of many organic reactions to be discussed, particularly in Chapter 11. The fundamental concept of a species donating electrons to a carbon is introduced in this section, with the goal of relating this chemical reactivity to the Lewis acid-Lewis base definitions used in previous sections. 

Although the Bronsted concept of acids and bases focuses on the transfer of a proton, electron pairs are more fundamental to the process. Covalent bonds are formed or broken when a proton is transferred from one atom to another. To account for this possibihty, Gilbert N. Lewis proposed a definition of acids that focuses on electron pairs. A Lewis acid is an electron pair acceptor a Lewis base is an electron pair donor. This is a general definition of an acid and a base. For example, hydrochloric acid, a Bronsted acid, is also a Lewis acid because it contains a proton that accepts an electron pair. Ammonia is a Lewis base because it can act as an electron pair donor. However, many other species can serve as electron pair acceptors or donors. Consider the following general reaction between a Lewis acid and a Lewis base. 

According to the Lowry-Brdnsted theory, a Brdnsted acid is a proton donor, while a Brdnsted base is a proton acceptor. In Lewis concept, acid acts as electron-pair acceptor, while base is electron donor (such as molecules possessing electton lone pairs). Hence, a Lewis base is in practice equivalent to a Brpnsted base. However, the concepts of acidity are markedly different [27]. 

Since Arrhenius, definitions have extended the scope of what we mean by acids and bases. These theories include the proton transfer definition of Bronsted-Lowry (Bronsted, 1923 Lowry, 1923a,b), the solvent system concept (Day Selbin, 1969), the Lux-Flood theory for oxide melts, the electron pair donor and acceptor definition of Lewis (1923, 1938) and the broad theory of Usanovich (1939). These theories are described in more detail below. 

As heavier analogs of carbenes141) stannylenes can be used as ligands in transition-metal chemistry. The stability of carbene complexes is often explained by a synergetic c,7t-effect cr-donation from the lone electron pair of the carbon atom to the metal is compensated by a a-backdonation from filled orbitals of the metal to the empty p-orbital of the carbon atom. This concept cannot be transferred to stannylene complexes. Stannylenes are poor p-a-acceptors no base-stabilized stannylene (SnX2 B, B = electron donor) has ever been found to lose its base when coordinated with a transition metal (M - SnXj B). Up to now, stannylene complexes of transition metals were only synthesized starting from stable monomoleeular stannylenes. Divalent tin compounds are nevertheless efficient cr-donors as may be deduced from the displacement reactions (17)-(20) which open convenient routes to stannylene complexes. 

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